This invention relates to dental posts, and more particularly to a dental post with intermittent threads which provide both active thread segments and passive land surfaces between the segments, to improve post retention within a prepared tooth stub on which a dental restoration will be erected, while maintaining tooth stress as a minimum.
Additionally, this invention relates to the use of the threaded active segments to provide thread-like impressions in the dentin of the tooth in the areas of the passive land surfaces of the post, which results in improved retention when a dental cement is used. Furthermore, this invention provides the benefits of the increased holding power of active surfaces while minimizing stress on the tooth stub. In one embodiment the thread segments of this invention extend only for a portion of the axial length of a post, and the remainder of the post is a passive surface containing contours for cement retention.
In restoring dentition, one procedure is to build up a dental prosthetic structure on a tooth stub. The tooth stub is initially prepared by cutting it down to provide a suitable support on which the prosthetic structure will be built. A bore is formed into the tooth stub in which a dental post is inserted. In one type of dental post, referred to as an active post, there are threads provided on the post and the post is threaded into the bore in the tooth stub. Other posts are referred to as passive posts and they are secured in the bore by means of cement. The passive dental post typically includes a contoured surface for improving its retention in the bore formed in the tooth stub. Suitable dental cement is used for retaining the dental post in the bore. Even active posts use some cement. However, in active posts the retention is mainly from the threads cutting into the dentin. A portion of the dental post extends upwardly above the surface of the tooth stub so that as the dental prosthetic structure is formed or built up onto the tooth stub, it is retained in place on the tooth stub by means of the extending portion of the dental post.
Various active and passive posts have been suggested in the prior art in order to improve the retention of a dental post within a bore in a tooth stub. By way of example, there has been suggested in U.S. Pat. Nos. 702,111; 4,515,565 and 4,846,685, posts with screw threads which are screwed into the root cavity of a tooth. Threaded engagement of a post into the dentin of a tooth has been determined to stress and weaken the tooth thereby contributing to its possible fracture. On the other hand, using passive posts produce less stress in the tooth. However, the retention of passive posts is not as great as that of active posts. To improve retention, various types of contoured surfaces have been suggested for the passive post. By way of example, U.S. Pat. No. 4,479,783, issued Oct. 30, 1984 for "Helically Fluted Dental Post", assigned to the assignee of the present invention, suggests contouring the surface of a passive dental post with helical flutes in a sequence with designated flutes being deeper than alternating shallow flutes to improve:retention of the dental post. Improvements in the capabilities of dental posts are also described in U.S. Pat. No. 4,729,736, issued Mar. 8, 1988 for "Contoured Dental Posts", assigned to the assignee of the present invention, including helical grooves and annular retaining ledges axially spaced along the post to both improve retention of the post and increase the strength of the post to reduce the possibility of post shear.
In a co-pending application, Ser. No. 303,900, filed Jan. 30, 1989, there is suggested the use of a dental post having an active threaded portion along an axial portion of the post and a passive portion axially along the rest of the post in order to receive the benefits of the increased holding power of a threaded active portion. In order to avoid stressing the tooth with the active portion, a sleeve with a threaded active portion is formed in a tooth stub. The threaded active portion of the post is then engaged with the threaded active portion of the sleeve to hold the post in the tooth stub. While the use of the threaded sleeve reduces stress, an additional step of forming the sleeve has been added to the dental restoration process.
As will be described, the present invention provides the benefits of active surfaces on a dental post while minimizing stress and increasing the bond between the tooth and the passive surfaces on the dental post.
While the aforementioned dental posts have provided improvements with respect to the retention of a post in a bore, still further improvements in such retention would be beneficial. These improvements would be particularly beneficial if they can be achieved without appreciably increasing the risk of breaking the dental post or the tooth.